It is necessary to temper the D2 steel as soon after the quench procedure as is practical. Do not let the steel grow cold after quenching by delaying the tempering procedure. There will be a serious risk of cracking, particularly if retained austenite is present. The retained austenite will begin its decomposition and transformation to untempered fresh martensite. This will exhibit an increase in hardness as well as a volumetric change (dimensional size change). In other words, the steel will (most likely) grow in size.
The tempering-temperature selection will depend on what the austenitizing temperature was in relation to the as-quenched hardness. The higher the austenitizing temperature selection and the longer the time, the more carbon, chromium and remaining alloys will have been taken into solution. This means that more carbides are available at elevated tempering temperatures. Tempering must always be performed at least twice. The purpose of this is that if there is any retained austenite, then at least 50% of that austenite will be decomposed at each subsequent temper. This also helps the steel for dimensional stability by the decomposition of the retained austenite.
The problem of retained austenite will usually be “observed” by the as-quenched hardness test. However, the “tree” in Figure 1 shows alternatives methods of determination. The probable causes of retained austenite are shown in Figure 2, and the probable causes of cracking are shown in Figure 3.
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